Nickel plating by chemical reduction



NEQKEL rLAriNo BY cnarviicAL REntJcrioN Edward L. Bolin, Zndianapolis, hath, assignor to Generai Motors Corporation, Detroit, Mich, a corporation of Delaware No Drawing. Application May 21, 1953 Serial No. 356,579

1 @iaim. (Cl. 117=65) This invention relates to improvements in the deposition of nickel from chemical reduction baths.

It is known that nickel can be deposited on certain types of metals by immersion of the metal in an aqueous solution or" a nickel salt and a reducing agent such as sodium hypophosphite or potassium hypophosphite. Such reduction plating baths are usually operated at a temperature of about 90 C. or higher in the presence of certain catalytic metals which initiate the oxidationreduction plating reaction. There are two general types of solutions, alkaline and acid, of which the acid solution is better adapted at present for commercial type plating operations. The most common metals on which nickel can be plated by the chemical reduction process are iron, steel, nickel, gold, cobalt, aluminum and palladium.

These metals have certain inherent properties which initiate the oxidation-reduction plating reaction. Other metals such as silver, copper, plantinum, and brass can also be plated by momentarily bringing a more electronegative metal such as aluminum or iron into contact with the surface of the metal while it is immersed in the plating solution.

Chemical reduction plating of nickel has certain inherent advantages such, for example, as the ability to provide a smooth uniform deposit of nickel on even the most irregularly shaped articles. However, up to the present time the use of chemical reduction type nickel plating has been limited to certain specialized applications Where conventional electroplating techniques did not provide satisfactory coatings. One of the most difficult problems in prior acid nickel reduction plating baths has been that of a rather limited bath operating life. Moreover, it has been found that in continued operation prior acid type chemical reduction plating baths do not provide a uniformly smooth nickel deposit, the roughness of the nickel deposit increasing as there is an accumulation in the bath of reaction products formed during the plating operation.

Accordingly, the principal object of the present invention is the provision of an improved acid type chemical r reduction plating bath for the deposition of nickel. A further object of the invention is the provision of a chemical reduction plating bath for the deposition of nickel coatings having a greatly improved corrosion resistance. A further object is the provision of an acid type chemical plating bath having an improved stability and useful operating life. A still further object of the invention is the provision of an improved reduction type plating bath and process for obtaining extremely smooth uniform deposits of nickel. V

I have discovered that an adherent nickel deposit having improved surface characteristics can be deposited form an aqueous bath comprising a nickel salt, a reducing agent capable of reducing the nickel salt to the free metal, and a buffer selected from the group consisting of glycollic acid, lactic acid, and acetic acid. I have found that these particular monohydroxymonocarboxylic and monocarboxylic acids provide a greatly improved pH stability and increased operating life of the plating solution. Excellent results have been obtained in accordance with this invention by employing a plating bath consisting essentially of an aqueous solution of a nickel salt, sodium hypophosphite, and a buffer selected from the group consisting of acetic acid, glycollic acid and lactic acid.

in many applications, superior results are obtained using baths of the aforementioned type containing a small amount of added gelatin or animal glue. Typically practicable amount of glue and/or gelatin to be used are additions of about .01 to .05 gram per liter of solutic-n, 0.2 gram per liter preferred at present. The use of the glue or gelatin additive provides a nickel deposit which is much more corrosion resistant than prior nickel coatings obtained from chemical reduction plating baths. Moreover, a nickel deposit obtained from a reduction plating bath employing the glue and/or gelatin additive of the present invention has an increased smoothness and luster.

in general, chemical reduction plating baths embodying the present invention have a composition Within the following range Where the quantities expressed are per liter of water:

Grams Soluble nickel salt 50 Reducing agent 5-100 Butler -100 The following are illustrative examples of particular bath compositions in accordance with the present invention in which the quantities are expressed per liter of water.

Example 1 Nickel chloride grams 30 Sodium hypophosphite do 10 70% glycollic acid milliliters 40 Example 2 Nickel chloride grams 35 Sodium hypophosphite do 10 85% lactic acid milliliters 50 Example 3 Nickel acetate grams Potassium hypophosphite do 15 Glacial acetic acid milliliters 40 E .ample 4 Nickel chloride grams 30 Sodium hypophosphite do 10 Glacial acetic acid milliliters Gelatin "grams" .02

Example 5 Nickel chloride grams Sodium hypophosphite c do 10 70% glycollic acid milliliters Animal glue grams .02

Nickel is deposited from the aforementioned plating baths preferably at a pH within the range of 4 to 6 although in certain applications, the pH range may extend from about 3 to 6.5. The bath generally may be operated at a temperature Within the range from about F. to 212 F, although a preferred operating range is approximately F. to 200 F. In general, the rate of nickel deposition is substantially proportional to the concentration of the reducing agent, approximately 2 grams of nickel being reduced by about 10 grams of hypophosphite. in many applications, plating baths embodying the present invention permit the deposition of nickel generally at a rate of about .0005 to .0006 inch v bath.

,rate of'nickel deposition in any particular application depends on several variables, in addition to the concentration of reducing agent, the-exact interdependence of which is not clearly understood at present.

' To obtain the best deposits of .nickel, it is essential that the-surface of the article to be coated is thoroughly clean. I A preferred cleaning procedure for steel or the like comprises the following steps: a vapor degrease treatment, an alkali electrolytic etch, water rinse, an acid etch, and 'water rinse prior to immersion in the plating In the preparation of plating baths embodying the invention the nickel salt preferably is first dissolved in Water and the buffer and reducing agent subsequently added. Superior results have been obtained by oi oi aging afresh solution before use,'or electrolyzing the fresh solution at a relatively low current density to plate out impurities which are introduced with the technical grade chemicals usually employed. 1 have discovered that've'ry beneficial results are obtained by adding a small amount of an oxidizing agent, such as hydrogen peroxide, preferably approximately 5 20 milliliters of 30% hydrogen peroxide per liter of plating solution, and maintaining the bath at an elevatedtemperature, preferably at least 90 C., for about 12 to 24 hours, thereafter introducing the required amount of reducing agent, and filtering the resultant solution to complete the preparation of the bath. By removing the harmful impurities, es-

peciallythe minute quantities of iron compounds which cause the deposition of highly stressed nickel, in'the aforementionedaging. and filtering treatment, commercial grade reagentscan be employed without danger of impairing the quality 'of the nickel deposit.

In continuous operation, chemical plating solutions require periodic replenishment of the nickel salt and reducing agent,as Well as pl-l adjustments. These ad ditions are preferably made separately in a water solution and filtered into the operating tank. The nickel salt and reducing agent additions can be made at any reduction plating operation in accordance with the pres.- ent invention. A plating bath is preparedby dissolving in water grams per liter of nickel chloride, 30' millilitersof glacial acetic acid, and. .02 gram per liter of gelatin. The resulting solution is then treated with about 10 milliliters per liter of 30% hydrogen peroxide and aged at C. for 24 hours. At the end of the aging period 10 grams per liter of sodium hypophosphite are added and theresultant solution filtered. A

carefully cleaned S. A. E. 1010 steel panel which is to be plated with nickel is then immersed in the bath which is maintained at a temperature of about 90 C. and'at a pH of 4.8 to 5.5, the pHadjustment being made by although git-is -preferable to make the pH adiustments 7 when the plating solution is cool. The pH adjustments usually are made in the form of a weak aqueous solution, about l% to 5%, of sodium hydroxide, potassium hydroxide, ammonium hydroxide, or other basic material 7 to avoid local precipitation of nickel salts. Foroptimum resultsthe' pH of the plating solution should be maintained within the range of 4.8 10 5.5 (colorimetric). v

I have found that analytical control of the chemical plating operation can be effectively provided by making frequent determinations of'the nickel salt content and maintaining thedesired bath com osition by appropriate additions of the nickel salt to the solution. inasmuch as about 2 grams of nickel reduced by 10 grams by the hypophosphite, the correct ,5. ite addition is readily determinable from the required nickel addition.

After the nickel has been deposited on a metal article was 3.1 4

'I have found that it'is advantageous to heat treat the nickel coating at an elevated temperature, typically at a temperature within'the range of about 350 F. to 400 F., preferably 47T E, for a period or" approximately 3 hours. Such aitr'ea ent not on embrittlement of the coating,"

-y removes any hydrogen improves the bond of the nickel to the base material as well as improving the ma'chinability of the deposited coating.

adding sodium hydroxide. The steel panel is emmersed in the solution for a time sufiicient to deposit the desired The nickel coated panel is then rethickness of nickel. moved from the bath, rinsed in water, and air. dried. To

improve the ductility and corrosion resistance the coated panel is then subjected to a heat treatment at a temperature of 750 F.for.2 hours; l a

From the foregoingdescription it now will be understood that lhave provided a chemical reduction plating bath which not only provides a uniformly smooth and lustrous deposit of. nickel but which additionally permits the deposition'of anickel coating having a greatly.

In some instances, the

improved corrosion resistance. corrosion resistance is as much as five times greater than thecorrosionresistance of nickel coatings deposited from prior chemical reduction plating baths.

his to be understood that, although the invention has been'described with specific reference to particular-'embodiments thereof, it is not to be so limited since changes and alterations therein maybe made which are within a of this invention as defined by the the full intended scope appended claim.

What is claimed is: b V

A process for plating nickelon a metalarticle which comprises the steps of providing an aqueous chemical reduction plating bath comprising, 5 to 50 grams per liter of a soluble nickel salt, 5 to grams per liter of a hypophosphite reducing agent, 15 to 100 grams per liter of a compound selected from the group consisting of glycollic acid, lactic acid, and acetic acid and .01 'to .05 gram per liter of a material selected from the group consisting of glueand. gelatin, adjusting the. pH of said bath to within the range of 3 to 6.5, heating said bath .to a temperature within the range of about amazon" F., and whileisaidbatli is maintained at a temperature I within this range, immersing the.metal article to be plated in the bath'until the desired thickness of nickel plate is deposited thereon, and thereafter subjecting the plated article to aheat treatment at a temperature within'the range of about 350 F. to 800 F. for a time of at least one hour to improve the ductility and corrosion resistance of the nickel plate.

References Cited in the file of patent I UNITED, STATES PATENTS 90,565 Lynd May 25, 7

Y 1,906,689. Ledig May 2, 1933 2,436,940 Sch ulein Mar. 2,.1948 2,501,737 Porter Mar. 28; 1950 2,624,684 Heiman Ian. 6," 1953 2,658,841 'Gutzeit Nov; 10, '1953 Gutzeit Nov. .10, 1.953

' OTHER REFERENCES Brenner et alJPart of the Journal of Research of the National Bureau of Standards, Research Paper RP1835, 'vol. 39, pages 385m 395 November 1947. 

